It's refreshing to see you folks not arguing about which OS is better (at least, not yet. xP) I am a mac user and I have been my entire life. I have a macbook with OS X.4, an iMac G3 with OS X.3 (The oooold kind, from like '00), a PerformaPC 6025CD (ANCIENT; has a 1 Gig harddrive and 4 MB of RAM, lulz.), a powerbook 500 (doesn't run; ~12 yrs old) and my uncle is sending us his old iMac G4 since he's getting a new one soon.
fluffums - your mom is probably reluctant to get a mac for 3 reasons:
1. Value. They are pretty overpriced for what you get. The hardware that comes with a Mac is pretty subpar compared to what you can get on a PC for the same amount of $, and you are locked in with some of it (e.g., you can't change out the vid card on a macbook because it's integrated into the motherboard.) That said, mac hardware components are 'tailored' to run well with each other and the OS. Still, the fact remains that you pay a premium for non-premium hardware.
2. Change. Your mum may just be used to MS XP, and if that's the case she probably sees no reason to change.
3. Misinformation & Opinion. A lot of PC users are pretty obsessed with forcing Microsoft propaganda down your throat (don't worry, Mac users are JUST as guilty, if not more as the minority.) PC users will often not hesitate to claim that MS is superior to OS X. Because Microsoft still holds the majority (although that is changing slowly as linux and mac popularity picsk up) most folks will tell your mom a PC is fine.